The present invention relates to semiconductor devices and more specifically to a semiconductor device having a magnetoresistive element.
As a semiconductor device, such as a semiconductor integrated circuit, for storage, DRAMs (Dynamic Random Access Memories) and SRAMs (Static Random Access Memories) have been conventionally in wide use. Meanwhile, MRAMs (Magnetic Random Access Memories) are a device (magnetoresistive element) that stores information by magnetism and has excellent features as compared with other memory technologies in high-speed operation, rewriting endurance, nonvolatility, and the like.
In conjunction with recent miniaturization of semiconductor integrated circuits, a contrivance may be given to the layout of memory elements in MRAM or the like as viewed in a plane. Specifically, the means disclosed in, for example, Japanese Unexamined Patent Publication No. 2010-219098 (hereafter, referred to as “Patent Document 1”) and Japanese Unexamined Patent Publication No. 2008-130995 (hereafter, referred to as “Patent Document 2”) are taken. That is, memory elements adjoining to one another in the direction in which wirings are extended are so arranged that they are not placed on identical wirings. Specifically, the straight lines coupling the individual memory elements (magnetoresistive elements) adjoining to one another are extended in a direction oblique to the direction in which wirings are extended. Further integration of magnetoresistive elements is promoted and variation in operating characteristics from recording element to recording element is reduced by adopting this configuration.
There are also the following types of MRAMs: STT (Spin Transfer Torque)-MRAMs and domain-wall-motion MRAMs. In the STT-MRAM, recorded information is written by spin injection using spin torque. In the domain wall motion MRAM, recorded information is written by using spin torque to move a domain wall. The STT-MRAM and the domain wall motion MRAM are spin torque written MRAMs having memory elements in which recorded information is read or written by a current passed through magnetoresistive elements themselves. In the spin torque written MRAM, there is demand to further reduce a current used to read/write information. This is because if this current is increased, it is necessary to increase the size of each switching element (transistor) making up a memory cell and this may incur increase in the plane area of the memory cell.
However, if an information rewriting current is reduced, there is a possibility that the following takes place: for example, when information recorded in another memory cell is read, recorded information in a (unintended) memory cell adjoining to the other memory cell is erroneously rewritten by a current passed at that time. Or, also when an entire integrated circuit is left standing and is not actively operated, there is similarly a possibility that recorded information in a memory cell is unintentionally rewritten by a minute electric current. In MRAMs, as mentioned above, reduction in a current used to read/write information from/to a magnetoresistive element and a characteristic to suppress read/write error in a magnetoresistive element are in the trade-off relation.
To improve this trade-off relation, it is desirable to take the means described in, for example, Japanese Unexamined Patent Publication No. 2004-296869 (hereafter, referred to as “Patent Document 3”) and Japanese Unexamined Patent Publication No. 2009-194210 (hereafter, referred to as “Patent Document 4”). That is, to improve the above trade-off relation, it is desirable to set the aspect ratio of each magnetoresistive element as viewed in a plane to a value that is not 1 and largely deviated from 1. That is, each magnetoresistive element is formed in such a shape, for example, an oblong shape or an oval shape, that it is longer in one direction than in another direction orthogonal to the one direction. US Published Application No. 2006/0120147A1 (hereafter, referred to as “Patent Document 5”) discloses the following technology: in MRAMs of such a type that information is written by the current-induced magnetic field of a word line, cross talk is reduced by meandering word lines to increase the distance between them and adjoining magnetoresistive elements.
[Patent Document 1]
    Japanese Unexamined Patent Publication No. 2010-219098[Patent Document 2]    Japanese Unexamined Patent Publication No. 2008-130995[Patent Document 3]    Japanese Unexamined Patent Publication No. 2004-296869[Patent Document 4]    Japanese Unexamined Patent Publication No. 2009-194210[Patent Document 5]    US Published Application No. 2006/0120147A1